A power supply apparatus is used in the related art in order to switch a pulsation input voltage obtained by full-wave rectifying an AC voltage supplied from an AC commercial power supply, generate a desirable DC voltage, and supply a load of an electrical appliance or the like with the DC voltage. The power supply apparatus includes a noise filter including a capacitor for the purpose of removing noises. For example, there are a number of power supply apparatuses including a power source cut-off detection circuit and a circuit for discharging a residual electric charge in order to safely deal with the residual electric charge accumulated on the capacitor of the noise filter, in a case where a plug of the power supply apparatus is unplugged from a plug outlet (for example, see Patent Literatures 1 and 2).
The power source cut-off detection circuit acquires the pulsation input voltage by full-wave rectifying the AC voltage supplied from the commercial power supply using two diodes, for example. Such a power source cut-off detection circuit generally detects the power source cut-off by detecting no pulsation input voltage or detecting that a voltage signal that smoothes the pulsation input voltage is lowered. After the power source cut-off is detected, a discharge circuit discharges the capacitor of the input filter via the two diodes and discharge resistances (for example, see Patent Literature 1, specification, paragraphs [0032] and [0036], and FIG. 2).
Incidentally, power from the commercial power supply may not be stably supplied due to a power failure, an instantaneous voltage drop, or the like. In such a case, an uninterruptible power supply (source) apparatus (UPS) may be used in order to stably supply the electrical appliance with power. FIG. 1 is a block diagram showing a configuration of a system using the UPS. An UPS 200 is connected between the commercial power supply and a power supply apparatus 101. When the power supply from the commercial power supply is interrupted, the UPS 200 supplies an electrical appliance 300 with the AC voltage generated from electrical energy accumulated in a battery of the UPS 200 in place of the AC voltage supplied from the commercial power supply.
A waveform of the AC voltage that is supplied to the electrical appliance 300 from the UPS 200 depends on products. FIG. 2A shows a sine wave of an AC voltage from a commercial power supply, for example. FIGS. 2B and C show illustrative pseudo sine waves output from the UPS 200, respectively. As shown in FIG. 2C, an UPS product of one example supplies the electrical appliance 300 with power by a rectangular wave voltage.